Production of Paraxylene

ABSTRACT

The process concerns ethylbenzene conversion and xylene isomerization with a catalyst pretreated by sulfiding.

PRIORITY CLAIM

This application claims the benefit of Provisional Application No.61/529,519, filed Aug. 31, 2011, the disclosure of which is incorporatedby reference in its entirety.

FIELD OF THE INVENTION

The invention relates to xylene isomerization and more particularly to amethod of treatment of a catalyst useful therefore and a start upprocedure for a xylene isomerization process.

BACKGROUND OF THE INVENTION

Paraxylene (also “p-xylene” or “PX”) is generally considered the mostimportant of C8 aromatic isomers, being used as an intermediate orstarting material for such diverse end uses as synthetic fibers andbottle plastic. Paraxylene is typically obtained from a C8 aromatichydrocarbon mixture derived from reformate by processes includingaromatic extraction and fractional distillation. Although thecomposition of this starting C8 aromatic hydrocarbon mixture varies overa wide range, the mixture generally comprises 5 to 40 wt % ethylbenzene,with the balance, xylenes, being divided between approximately 50 wt %meta-xylene and 25 wt % each of para-xylene and ortho-xylene (thisdistribution considered the nominal “equilibrium concentration” ofxylenes). Since, by some accounts, 80 wt % or more of the end use ofxylenes involves the conversion of para-xylene to the above-mentionedend uses, obtaining para-xylene from its C8 isomers meta-xylene,ortho-xylene, and ethylbenzene, is the subject of a vast amount ofcontinuing research.

By way of example, U.S. Pat. No. 5,004,855 teaches a process for theconversion of ethylbenzene in an aromatic hydrocarbon mixture comprisingplacing a C8 aromatic hydrocarbon mixture containing ethylbenzene andxylenes in the presence of hydrogen and in contact with a catalystcomprising rhenium, an acid type of a zeolite having a main cavity inletcomposed of a 10-membered oxygen ring, and alumina, said catalyst havingbeen subjected to a sulfiding treatment, to effect conversion ofethylbenzene to benzene. In embodiments, the ethylbenzene conversionstep is conducted prior to and separately from the passage of the feedthrough the circulation system including para-xylene separation andxylene isomerization.

In U.S. Pat. No. 5,516,956 a mixture of aromatic hydrocarbons,comprising ethylbenzene and at least one xylene, is isomerized using atwo component catalyst system to convert the ethylbenzene to compoundsthat may be removed from the aromatic hydrocarbon stream and to producea product stream wherein the para-xylene concentration is approximatelyequal to the equilibrium concentration of the para-isomer. The firstcatalyst comprises an intermediate pore size zeolite that is effectivefor ethylbenzene conversion, and the second catalyst comprises anintermediate pore size zeolite, which further has a small crystal sizeand which is effective to catalyze xylene isomerization. Each of thecatalysts of this invention may contain one or more hydrogenationcomponents.

In U.S. Pat. No. 6,028,238, a process is described for isomerizing afeed which contains ethylbenzene and xylene, which process comprises thesteps of: (a) contacting the feed under ethylbenzene conversionconditions with a particulate first catalyst component which comprises amolecular sieve having a constraint index of 1-12, the particles of saidfirst catalyst component having a specified surface to volume ratio andthe contacting step converting ethylbenzene in the feed to form anethylbenzene-depleted product; and then (b) contacting theethylbenzene-depleted product under xylene isomerization conditions witha second catalyst component.

Sulfur modification of a xylene isomerization catalyst is taught in U.S.Pat. No. 7,271,118. The catalyst comprises a Group VIII metal (referringto the traditional “CAS version” of the Periodic Table).

In prior art processes such as in the above-mentioned U.S. Pat. No.6,028,238, a paraxylene-depleted C8 aromatics feed (meaning that theamount of paraxylene is less than the equilibrium concentration referredto above) is contacted with a catalyst system that de-alkylatesethylbenzene to benzene while isomerizing the xylenes to an equilibriummixed xylene product. The ethylbenzene dealkylation and xyleneisomerization reactions are advantageously accomplished in a dual-bedcatalyst system. However, in commercial practice such units oftenexperience large start-up exotherms during the initial oil-in period(contact of the catalyst with feed). An extreme bed temperatureexcursion can occur particularly when the liquid feed pump is incapableof delivering the hydrocarbon flow rate to a full design capacity withina short period of time. The resulting high hydrogen to hydrocarbon(H₂/HC) molar ratio and high hydrogen partial pressure promotehydro-dealkylation and hydrogenolysis reactions, catalyzed by thehydrogenation metal component in the catalyst system, which in turncauses excess heat of reaction. Such a start-up exotherm can lead, amongother negative consequences, to premature unit shutdown, mechanicalfailure of the equipment, poor isomerization performance, reducedcatalyst life, and loss of xylenes, in commercial applications.

It is desirable to mitigate the start-up exotherm, so as to avoidnegative consequences and maintain the high performance characteristicsof the xylene isomerization catalyst.

The present inventors have surprisingly discovered a catalystpre-treatment and start-up procedure that overcomes the disadvantages ofthe prior art system.

SUMMARY OF THE INVENTION

The invention concerns mitigation of large exotherms at the start-up ofa process for the production of paraxylene comprising contact of a C8aromatic hydrocarbon feed with a catalyst system, by sulfiding of thecatalyst prior to contact with the feed.

In embodiments, the invention includes treating a dried and reducedcatalyst comprising a hydrogenation metal so as to sulfide at least aportion of the metal. In preferred embodiments, the catalyst is treatedwith diluted H₂S gas at an elevated temperature and pressure prior tooil-in.

It is an object of the present invention to provide a process forconverting ethylbenzene to benzene and the isomerization of xylenes in aprocess for the production of paraxylene from a mixture of C8 aromatichydrocarbons using a catalyst comprising a hydrogenation metal thatavoids negative consequences of a high exotherm during start upprocedures.

Another object of the present invention is to provide a process for theproduction of para-xylene including the conversion of ethylbenzene tobenzene in a C8 aromatic hydrocarbon mixture with a reduced loss ofxylene.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a temperature profile of the catalyst bed showing no start upexotherm using a catalyst according to the present invention.

FIG. 2 is a temperature profile of the catalyst bed showing a largestart-up exotherm using a freshly reduced catalyst.

FIG. 3 shows a performance comparison between the presulfided catalystand the freshly reduced catalyst with respect to dealkylation activity.

FIG. 4 shows a performance comparison between the presulfided catalystand the freshly reduced catalyst with respect to PX selectivity.

FIG. 5 shows a performance comparison between the presulfided catalystand the freshly reduced catalyst with respect to xylene loss.

DETAILED DESCRIPTION

The present invention concerns improved manufacture of paraxylene from aC8 aromatic hydrocarbon stream by a process that includes mitigation oflarge exotherms at the start-up of said process by sulfiding of thecatalyst prior to contact with the feed (“presulfiding”).

In embodiments, the method comprises treating the dried and reducedcatalyst with diluted H₂S gas at an elevated temperature and pressure.

Without wishing to be bound by theory, it is believed that H₂S moleculesare chemically adsorbed on the surface of the metal component of theisomerization catalyst to form a layer of metal sulfides. Such metalsulfides have less or no activity in aromatic ring saturation and alkanehydrogenolysis. Thus, when hydrocarbons are introduced into the reactorsystem, the reaction exotherm is minimized However, the metal sulfidesare not stable in the strong reducing environment following contact withthe aromatic hydrocarbon feed (“on-oil”), so passivation of the catalystmetal function is of short duration and the activity suppressing orpoisoning effect is reversible. Hence, the metal sulfides formed in thepretreatment step will be reduced and sulfur will desorb from thecatalyst surface following aromatic hydrocarbon introduction undercontinuous hydrogen gas circulation at elevated temperature andpressure. Thus the long-term catalyst activity and selectivity will notbe adversely affected.

The catalyst components including a hydrogenation component, such asprovided by one or more metals selected from Group 7 (e.g., rhenium) orGroups 8-10 (e.g., platinum; formerly “Group VIII”), using the moderndesignations of the groups in the Periodic Table, a molecular sieve, anda support such as alumina or clay.

In preferred embodiments the hydrogenation component is rhenium (Re); inother embodiments the catalyst is not pre-selectivated such as withsilica (although it may be steam-treated, as discussed in more detailbelow), in other embodiments the catalyst is molded by known methodsknown in the art per se, such as extrusion molding, compression molding,and rolling moldings. The embodiments may be combined so that, by way ofexample, a preferred embodiment is a catalyst comprising Re where thecatalyst has not been selectivated with silica and is provided in moldedform prior to sulfiding.

In general, the sulfiding used may be any method capable of convertingthe hydrogenation component to a sulfide in a current of hydrogensulfide at from above room temperature to 540° C., preferably 100° C. to450° C. The time of the sulfiding treatment is not particularlycritical, however, the treatment should be conducted after thehydrogenation component has been supported on the catalyst, methods ofwhich are per se known in the art. Preferred methods of applying saidcomponent to the catalyst include impregnation, ion exchange, or mixing.

In general, the feedstock may comprise an aromatic C8 mixture containingethylbenzene and at least one xylene isomer and typically all three ofthe xylene isomers. In embodiments the feedstream will be para-xylenedepleted, meaning that the concentration of para-xylene in thefeedstream, relative to its C8 isomers, will be lower than thethermodynamic equilibrium concentration of para-xylene in a mixture ofC8 isomers. In embodiments the feedstream will have an ethylbenzenecontent in the approximate range of 5 to 60 wt %, an ortho-xylenecontent in the approximate range of 0 to 35 wt %, a meta-xylene contentin the approximate range of 20 to 95 wt % and a para-xylene range of 0to 15 wt %.

In addition, the feedstream comprising C8 aromatic hydrocarbons maycontain non-aromatic hydrocarbons, e.g., naphthenes and paraffins, suchas in an amount up to 30 wt %.

In a preferred embodiment, the invention provides a method to process astarting mixture comprising C8 aromatic hydrocarbons, such as thatderived from catalytic reforming of a petroleum naphtha, to obtain aproduct mixture of C8 aromatic hydrocarbons having a reducedethylbenzene content and increased paraxylene content relative to saidstarting mixture, said method having at least one advantage selectedfrom longer catalyst life, longer on-oil time, reduced xylene loss, andincreased paraxylene recovery. The invention is particularly effectivein treating a paraxylene lean mixture of C8 aromatic hydrocarbons toincrease the paraxylene concentration up to approximately the thermalequilibrium level.

The process of the present invention is especially suitable for theisomerization of C8 aromatic hydrocarbon streams that contain about 5 to60 wt % ethylbenzene, e.g., about 8 to 15 wt % ethylbenzene. This rangespans the range of ethylbenzene concentrations of streams that arederived from a reformer and a pyrolysis gasoline unit. In certainembodiments, the presulfided catalyst of the present invention isbelieved to have the advantage of high activity for cracking of normaland branched paraffins of the type present in unextracted C8 aromaticstreams.

The catalyst system of the invention includes at least two catalystcomponents, the first of which has the primary function of selectivelyde-ethylating the ethylbenzene in the feedstream to benzene, while thesecond catalyst component selectively isomerizes xylenes in the feed.The first catalyst component can, and preferably will, effect someisomerization of the xylenes in the feed. Such catalyst systems and therespective components are per se known in the art and can be selected byone of ordinary skill in the art in possession of the presentdisclosure. Particularly preferred systems are disclosed, for instance,in the aforementioned U.S. Pat. Nos. 5,516,956 and 6,028,238, mindful ofthe teachings of the present disclosure.

In embodiments, each of the first and second catalyst componentscomprises an intermediate pore size molecular sieve which ischaracterized by a Constraint Index within the approximate range of 1 to12 (e.g., less than about 7 Angstroms pore size, such as from about 5 toless than about 7 Angstroms). The term “Constraint Index” is well-knownin the art. The method by which Constraint Index is determined isdescribed fully in U.S. Pat. No. 4,016,218, incorporated herein byreference for details of the method. Examples of intermediate pore sizemolecular sieves useful in this invention include ZSM-5 (U.S. Pat. Nos.3,702,886 and Re. 29,948); ZSM-11 (U.S. Pat. No. 3,709, 979); ZSM-12(U.S. Pat. No. 3,832,449, ZSM-22 (U.S. Pat. No. 4,556,477); ZSM-23 (U.S.Pat. No. 4,076,842); ZSM-35 (U.S. Pat. No. 4,016,245); ZSM-38 (U.S. Pat.No. 4,406,859); ZSM-48 (U.S. Pat. No. 4,397,827); ZSM-57 (U.S. Pat. No.4,046, 685); and ZSM-58 (U.S. Pat. No. 4,417,780).

The molecular sieve of each of the first and second catalyst componentsis also associated with at least one hydrogenation component. Examplesof such components include the oxide, hydroxide, sulfide, or free metal(i.e., zero valent) forms of the metals discussed above (i.e., selectedfrom Groups 7-10) and also from Group 6 (i.e., Cr, Mo, W), Group 11(i.e., Cu, Ag, Au), Group 14 (i.e., Sn and Pb), and Group 15 (i.e., Sband Bi). Combinations of catalytic forms of such noble or non-noblemetal, such as combinations of Pt with Sn, may be used. The metal ispreferably in a reduced valence state, e.g. when this component is inthe form of an oxide or hydroxide. The reduced valence state of thismetal may be attained by methods per se known in the art, e.g., in situ,such as when a reducing agent, e.g., hydrogen, is included in the feedto the reaction. In preferred embodiments the metal is rhenium and it ispresent in the reduced state in the dried catalyst prior to sulfiding.

As far as incorporation of the hydrogenation component into thecatalyst, it is preferred that said component be incorporated into thecatalyst by ion exchange, impregnation or physical admixture. Forexample, solutions of appropriate metal salts may be contacted with theremaining catalyst components (molecular sieve and support such asalumina and/or clay), which have preferably not been selectivated withsilica, under conditions sufficient to combine the respectivecomponents. The metal containing salt is preferably water soluble.Examples of suitable salts include perrhenic acid (HReO4). Suitablemethods of preparation also include the use of aqueous solutions ofrhenium oxides, such as disclosed in U.S. Pat. No. 5,004,855. Afterincorporation of the metal, the catalyst can then be filtered, washedwith water and calcined at temperatures of from about 250 to about 500°C. In embodiments the thus-prepared dried and calcined catalyst is thenloaded in a reactor, dried, and then reduced, such as under flowinghydrogen, prior to sulfiding.

The amount of the hydrogenation component is suitably from about 0.001to about 10 percent by weight, e.g., from about 0.1 to about 5 percentby weight, e.g., from about 0.1 to about 2 percent by weight, althoughthis will, of course, vary with the nature of the component and otherfactors, as would be appreciated as well as optimized by one of skill inthe art in possession of the present disclosure.

The above-described preparation of the dried and calcined catalyst,comprising molecular sieve, hydrogenation component, and optionalsupport such as by alumina and/or clay, does not form a necessary partof the present invention per se and is within the skill of the ordinaryartisan to prepare by no more than routine experimentation.

In practicing the process of the invention, it may be desirable toformulate either or both of the first and second catalyst componentswith another material resistant to the temperature and other conditionsof the process, e.g., a support or matrix. Such matrix materials includesynthetic or naturally occurring substances as well as inorganicmaterials such as clay, silica, and/or metal oxides. The metal oxidesmay be naturally occurring or in the form of gelatinous precipitates orgels including mixtures of silica and metal oxides. Naturally occurringclays which can be composited with the molecular sieve include those ofthe montmorillonite and kaolin families, which families include thesubbentonites and the kaolins commonly known as Dixie, McNamee, Georgiaand Florida clays or others in which the main mineral constituent ishalloysite, kaolinite, dickite, nacrite or anauxite. Such clays can beused in the raw state as originally mined or initially subjected tocalcination, acid treatment or chemical modification.

In addition to the foregoing materials, the molecular sieves employedherein may be composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-berylia, silica-titania, as well as ternary compounds such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia,and silica-magnesia-zirconia. A mixture of these components could alsobe used. The matrix may be in the form of a cogel. The relativeproportions of molecular sieve component and inorganic oxide gel matrixon an anhydrous basis may vary widely with the molecular sieve contentranging, in embodiments, from between about 1 to about 99 percent byweight and more usually in the range of about 10 to about 80 percent byweight of the dry composite.

The first and second components of the catalyst system of the inventionshould be selected so that the respective components differ from eachother in a number of significant respects which ensure that firstcomponent selectively deethylates the ethylbenzene in the feedstream tobenzene while the second component selectively isomerizes xylenes in thefeed. While selection of details of the catalyst to be sulfided arewithin the skill of the ordinary artisan in possession of the presentdisclosure, certain preferred characteristics are discussed below.

In embodiments, the first and second components of the catalyst systemof the invention differ in their particulate form and size. The firstcatalyst component is preferably composed of particles having a surfaceto volume ratio of about 80 to about 200 inch⁻¹, whereas the secondcatalyst component will typically be composed of particles with asurface to volume ratio less than 80 inch⁻¹.

Ethylbenzene Conversion Component: according to embodiments of theinvention, the first catalyst component, which selectively deethylatesthe ethylbenzene in the feedstream to benzene, is selected so as to havea surface to volume ratio of about 80 to <200 inch⁻¹, preferably about100 to 150 inch⁻¹. It has been found that the ethylbenzene conversionreaction is sensitive to intraparticle (macroporous) diffusionlimitations. By selecting the shape and size of the particles of thefirst catalyst component such that the surface to volume ratio is withinthe specified range, it is found that the intraparticle diffusiondistance can be decreased without excessively increasing the pressuredrop across the first catalyst bed. As a result, the xylene lossesaccompanying the ethylbenzene conversion in the first catalyst bed canbe reduced, while at the same time the xylene isomerization activity ofthe first catalyst component can be increased. Producing a firstcatalyst component with the desired surface to volume ratio can readilybe achieved by controlling the particle size of the catalyst or by usinga shaped catalyst particle, such as the grooved cylindrical extrudatedescribed in U.S. Pat. No. 4,328,130 or a hollow or solid polylobalextrudate as described in U.S. Pat. No. 4,441,990, the entire contentsof both of which are incorporated herein by reference. For example, acylindrical catalyst particle having a diameter of 1/32 inch and alength of 3/32 inch has a surface to volume ratio of 141, whereas aquadralobed solid extrudate having the external shape disclosed in FIG.4 of U.S. Pat. No. 4,441,990 and having a maximum cross-sectionaldimension of 1/16 inch and a length of 3/16 inch has a surface to volumeratio of 128. A hollow tubular extrudate having an external diameter of1/10 inch, an internal diameter of 1/30 inch and a length of 3/10 inchhas a surface to volume ratio of 136.

In embodiments, the first catalyst component preferably has enhancedmacroporosity which is achieved by adding a thermally decomposibleorganic material to the mix used to extrude the catalyst particles; andthen calcining the extruded particles to remove the organic material.The thermally decomposible organic material can be any material which iscompatible with the extrudable mix used to form the catalyst particlesand which is retained within the mass of the extruded catalyst particlesbut which can be removed from the catalyst particles by heating to leavemacroporous voids within the particles. A suitable organic material is acellulose such as that sold under the trade name Avicel.

In embodiments, the molecular sieve of the first catalyst componentpreferably has a higher acid activity (and thus a higher alpha value)than the molecular sieve of the second catalyst component. Thusmolecular sieve of the first catalyst component preferably has an alphavalue of at least 50 and typically has an alpha value of about 100 toabout 500.

Most preferably, the alpha value of the molecular sieve of the firstcatalyst component is between 100 and 300. The alpha test is describedin U.S. Pat. No. 3,354,078; in the Journal of Catalysis, Vol. 4, p. 527(1965); Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980), eachincorporated herein by reference as to that description. Theexperimental conditions of the test used herein include a constanttemperature of 538° C. and a variable flow rate as described in detailin the Journal of Catalysis, Vol. 61, p. 395. The higher alpha valuescorrespond with a more active cracking catalyst. Modification of thecatalyst acidity can be achieved by contacting the catalyst with hightemperature steam. The desired level of acidity (alpha values) can beachieved by controlling steaming temperature and duration, in a mannerwell-known to one of ordinary skill in the art.

In embodiments, each of the components of the catalyst system of theinvention will normally exhibit mutually exclusive xylene diffusionalproperties. These properties can be identified by noting the time (inminutes) required to sorb 30% of the equilibrium capacity ofortho-xylene at 120° C. and at an ortho-xylene partial pressure of4.5+-0 8 mm of mercury, a test described in U.S. Pat. Nos. 4,117,026;4,159,282; and Re. 31,782. The equilibrium capacity of ortho-xylene isdefined herein as greater than 1 gram of xylene(s) per 100 grams ofmolecular sieve. In the catalyst system of the invention, the firstcatalyst component effective for ethylbenzene conversion preferably hasan ortho-xylene sorption time (in minutes) in excess of about 50 andpreferably greater than about 1200, but less than 10,000 minutes, whileon the other hand, the second, isomerization component preferably has anortho-xylene sorption time of less than about 50 minutes and preferablyless than about 10 minutes.

In embodiments, the xylene diffusion properties of the first catalystcomponent is achieved in a number of ways. For ortho-xylene diffusiontimes at or near the minimum value of 50 minutes, the selection of alarge crystal form of the molecular sieve used in the catalyst, that ishaving an average crystal size in excess of 1 micron, may be sufficient.

The second component of the catalyst system is effective to isomerizethe xylenes of the feed containing C8 aromatics. The second,isomerization component preferably has an ortho-xylene sorption time ofless than about 50 minutes and preferably less than about 10 minutes.This is typically achieved by using a small crystal size molecularsieve, having an average crystal size of 0.02-0.05 micron, in thiscomponent. The molecular sieve of the second component of the catalystsystem will typically have an alpha value less than about less than 50and preferably from about 5 to about 25. The second component of thecatalyst system may be prepared with the use of a thermally decomposibleorganic material so as to increase its macroporosity. In addition, thesize and shape of the particles of the second catalyst component can beselected so as to have a surface to volume ratio of about 80 to <200inch⁻¹, preferably about 100 to 150 inch⁻¹.

After preparation as set forth above, the catalyst is sulfided inaccordance with the present invention. Preferably the catalyst issulfided in situ. In embodiments, the treatment comprises flowinghydrogen which contains 100-600 vppm H₂S gas at elevated temperaturesuch as from above room temperature to about 500° C., preferably 100° C.to 450° C. Normally liquid DMDS (dimethyl disulfide) is used assulfiding agent. DMDS decomposes to H₂S and methane once entering thereactor. The extent of sulfiding is preferably selected to obtain 0.5 to3.0 equivalents of catalyst metal content.

Conditions such as WHSV and H₂:HC at initial oil-in can be determined byone of ordinary skill in the art in possession of the present disclosurewithout more than routine experimentation. In this regard, it is usefulto keep in mind that in a commercial plant, typically the liquid feedpump is incapable of delivering the hydrocarbon flow rate to a fulldesign capacity within a short period of time. This results in low WHSVand high hydrogen to hydrocarbon molar ratio and high hydrogen partialpressure, which promote hydro-dealkylation and hydrogenolysis reactionscatalyzed by hydrogenation component such as Re, leading to excess heatof reaction.

When the presulfided catalyst system according to the present inventionis contacted with the feedstream, the conditions used in the process ofthe invention are not narrowly defined, but generally will include atemperature of from about 400 to about 1,000° F. (about 204° C. to about537° C.), a pressure of from about 0 to about 1,000 psig (6.895 MPa-g),a weight hourly space velocity (WHSV) of between about 0.1 and about 200hr⁻¹, and a hydrogen, H₂, to hydrocarbon, HC, molar ratio of betweenabout 0.2 and about 10. Preferably, the conditions include a temperatureof from about 650 to about 850° F. (about 340-450° C.), a pressure offrom about 50 and about 400 psig (about 0.34 to 2.76 MPa-g), a WHSV ofbetween about 3 and about 50 hr⁻¹ and a H₂ to HC molar ratio of betweenabout 1 and about 5.

In general, the process of the invention is carried out in a fixed bedreactor containing the catalyst system described above. In a preferredembodiment, the first and second components of the catalyst system arein sequential beds in a single reactor. That is, the component of thecatalyst system used in the process of the invention which is effectivefor ethylbenzene conversion forms a first bed, while the other componentof the catalyst system, which is effective for xylene isomerization,forms a second bed downstream of the first bed. The feed is preferablycascaded from the first to the second bed without intervening separationof light gases. As an alternative, the first and second beds could bedisposed in separate reactors which, if desired, could be operated atdifferent process conditions. Additional catalyst beds may be providedprior or after the first and second catalyst components of theinvention.

After the conversion process, the isomeration product can be treated toisolate paraxylene and/or other desirable xylene(s). Thus, for example,the isomerate product can be fed to a variety of paraxylene recoveryunits, such as a crystalizer, a membrane separation unit, or a selectiveadsorption unit (e.g., Parex™ unit), and thus the paraxylene may beisolated and recovered, leaving a paraxylene depleted C8 aromatichydrocarbon by-product or residual isomerate. The residual isomerate canbe stripped of products lighter than C8 aromatic hydrocarbons. Productsheavier than C8 aromatic hydrocarbons in the residual isomerate can befurther processed or may be fractionated out. C8 aromatic hydrocarbonfractions from which para-xylene has been removed can be recycled to theprocess.

The success of this method has been demonstrated in both pilot plant andcommercial units. The start-up exotherms were successfully minimized,which led to improved catalyst performance. The following examples aremeant to be illustrative of the present invention and not a limitationthereon. One of ordinary skill in the art in possession of the presentdisclosure will recognize that the invention may be practiced other thanas specifically illustrated herein below.

EXAMPLE 1

The experiments were carried out in a pilot plant scale fixed bed unit.The reactor has a catalyst basket with 0.64″ ID and 17.5″ length. Thebasket was loaded with xylene isomerization catalyst extrudates. The topbed is 1/20″ quadrolobe shaped extrudates which contain Re supported onsaid solid extrudates composed of ZSM-5 molecular sieve and a binder.The bottom bed is 1/16″ cylindrical extrudates which contain Re on saidsolid extrudates composed of ZSM-5 molecular sieve and a binder. Thedual-bed catalysts were packed in the reactor basket with a 35/65 bedratio, by weight (first catalyst: second catalyst). The total freshcatalyst loading was 15.5g. The reactor voids were filled with inertglass beads, and 80/120 mesh quartz sand was used to fill catalyst bedinterstitial spaces. These measures were taken to minimize channeling ofthe reactant gas flow. The reactor is equipped with a thermal well (⅛″OD), to allow a traveling thermocouple to record the catalysttemperature along the bed axis, so that an average reactor temperature(ART) can be obtained.

The feedstock was commercial grade para-depleted mixed xylenes, whichcontained 16.2 wt % ethylbenzene, 1.9 wt % p-xylene, 15.3 wt % o-xylene,64.7 wt % m-xylene, and 1.2 wt % toluene.

The catalysts were activated with temperature programmed reduction underhydrogen flow, followed by a metal passivation step via sulfiding.

When the catalyst was pre-sulfided, the reactor bed was held at thefinal reduction temperature (365° C.) before introducing H₂S stream. Asulfur containing gas mixture (H₂ and H₂S) was used in the sulfidingprocess. The H₂S concentration in the gas mixture was 400 vppm. Thesulfiding gas flowrate and sulfiding time were set to give 1.9-foldcoverage of rhenium atoms stoichiometrically (the chemisorption assumedto form ReS₂ compound). H₂S breakthrough (>100 vppm H₂S) was detected atthe reactor outlet by a Draeger tube.

The unit was subsequently started-up with feed introduction at a lowweight hourly space velocity (WHSV) of 5.4 hr⁻¹, which is about 20-50%of design capability and high hydrogen to hydrocarbon molar ratio(H₂/HC) of 9:1, simulating commercial unit startup conditions. The feedrate was increased gradually while H₂/HC was decreased.

FIG. 1 shows the catalyst temperature profiles at multiple points alongthe bed length during the oil-in period. No exotherm was observed duringthe entire oil-in period. As a direct comparison, when the catalyst wasnot sulfided, and otherwise under the same conditions, the initial feedintroduction resulted in a large exotherm, as shown in FIG. 2.

FIGS. 3, 4 and 5 show the performance comparison between the presulfidedcatalyst according to the present invention and the freshly reducednon-presulfided catalyst. The presulfided catalyst showed comparableethyl benzene (EB) dealkylation activity (FIG. 3), higher paraxyleneapproach to equilibrium (PXAE) (FIG. 4), and comparable or slightlylower xylene loss (FIG. 5). In addition, an approximately 30-day agingtest demonstrated that the presulfided catalyst showed the same agingrate as that of non-sulfided catalyst. This verified that thepresulfiding did not create any negative impact on the catalystlong-term performance.

EXAMPLE 2

In commercial unit A, the presulfiding method according to the inventionwas implemented. The sulfiding agent used was dimethyl-disulfide (DMDS)liquid. The injection facilities consisted of a storage barrel of DMDSon a weight scale, two positive displacement pumps, and associatedstainless steel tubing. Pre-sulfiding of the fresh xylene isomerizationcatalyst started immediately following completion of the catalystdry-out. The catalyst bed was maintained at an elevated temperature of359° C.

DMDS decomposed to H₂S and CH₄ once it was injected into the reactor.The DMDS injection rates were set to give 500 vppm (sometime termedppmV, or parts per million by volume) H₂S at the reactor inlet. Therecycle gas was monitored for H₂S breakthrough via Draeger tubes. Thetotal sulfur injected corresponded to 1 equivalent of catalyst metalcontent. Breakthrough of H₂S (>100 vppm in recycle gas) was detectedduring this commercial catalyst presulfiding application. No temperaturerise was detected in the reactor during DMDS injection.

Oil-in began immediately following pre-sulfiding. The initial feed ratewas at 65% of design, and the initial H₂/HC ratio was approximately1.8:1. The wetting exotherm was very mild compared to previous freshcatalyst start-ups. The exotherm (maximum bed T−inlet T) was 10-17° C.,with an observed maximum bed temperature of 371° C. This compares to themaximum temperatures during previous start-ups of 590-700° C., when thecatalyst was not sulfided.

Table 1 shows the performance comparison between the presulfidedcatalyst and the non-sulfided catalyst with start-up exotherm at similaroperating conditions and time-on-stream. The presulfided catalyst showedmuch higher EB dealkylation activity as measured by the normalizedaverage reactor temperature (NART), higher paraxylene approach toequilibrium (PXAE), and comparable xylene loss.

TABLE 1 Commercial Unit A performance comparison between the presulfidedcatalyst and the freshly reduced catalyst. Normalized Average ParaXyleneReactor Temperature*, Approach Xylene Loss, Catalyst ° C. Equilibrium, %% Presulfided, No 410 103.9 1.6 startup exotherm Freshly reduced, 462100.1 1.6 Large startup exotherm *Normalized to 15 WHSV, 1.7H₂/HC, 55%EBC (Ethyl Benzene Conversion)

EXAMPLE 3 (Commercial Unit B)

In commercial unit B, independent DMDS injection facilities were used tointroduce sulfur to two parallel reactors simultaneously as soon as thecatalyst dry-out and reduction processes were completed. Thepresulfiding was conducted at the reactor temperature of 360° C., andthe inlet pressure of 1.3 MPa. The DMDS injection rates were set to give500 vppm H₂S concentration at the reactor inlet. Recycle gas H₂Sconcentration was monitored frequently during the presulfiding process.Hydrocarbon feed introduction started immediately after the 2× sulfurequivalents (based on metal atom content) were injected. No H₂Sbreakthrough was detected in the recycle gas. Without wishing to bebound by theory, it is believed that the large metal surface area of thereactors and the feed/effluent heat exchangers had adsorped additionalsulfur.

The initial feed rate was at 28% of the design. Low WHSV relative todesign gives high H₂/HC at constant recyle gas rate. Both typically willlead to more exotherm. The H₂/HC ratio was at approximately 6. A verysmall wetting exotherm (<5° C.) was observed following oil-in. Thiscompares to large reactor bed temperature increases of 120-135° C.during previous start-ups.

Table 2 shows the performance comparison between the presulfidedcatalyst and the non-sulfided catalyst with start-up exotherm. Thepresulfided catalyst showed higher EB dealkylation activity (low NART),much higher p-xylene approach equilibrium (PXAE), and comparable orlower xylene loss.

TABLE 2 Commercial Unit B performance comparison between the presulfidedcatalyst and the freshly reduced catalyst. Normalized Average ParaXyleneReactor Temperature*, Approach Xylene Loss, Catalyst ° C. Equilibrium, %% Presulfided, No 395 101.0 0.65 startup exotherm Freshly reduced, 40593.0 0.73 Large startup exotherm *Normalized to 18 WHSV, 1.5H₂/HC, 40%EBC

In light of the above description and having provided numerous detailsincluding preferred embodiments and specific example, it will beunderstood that various other modifications will be apparent to and canbe readily made by those skilled in the art without departing from thespirit and scope of the invention.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

What is claimed is:
 1. A process for preparing paraxylene comprising:(a) contacting a C8 aromatic hydrocarbon mixture including ethylbenzeneand at least one xylene isomer other than paraxylene, in the presence ofhydrogen and under suitable ethylbenzene de-alkylating conditions, withat least a first catalyst comprising at least a first hydrogenationcomponent, wherein said hydrogenation component is selected from Groups6-11 and 14-15 of the Periodic Table, preferably rhenium, said componentsupported on a molecular sieve, wherein said catalyst is suitable forde-alkylation of ethylbenzene and further characterized as having beensubjected to a sulfiding treatment prior to said contacting, to producea ethylbenzene-depleted aromatic hydrocarbon mixture; then (b)contacting said ethylbenzene-depleted C8 aromatic hydrocarbon mixture,in the presence of hydrogen and under suitable xylene isomerizationconditions, with at least a second catalyst comprising at least a secondhydrogenation component, preferably rhenium, said component supported ona molecular sieve, wherein said catalyst is suitable for xyleneisomerization and further characterized as having been subjected to asulfiding treatment prior to said contacting, to produce aparaxylene-enriched C8 aromatic hydrocarbon mixture, when compared withsaid C8 aromatic hydrocarbon mixture of step (a).
 2. The process ofclaim 1, wherein said sulfiding treatment in at least one of steps (a)and (b) is carried out in a hydrogen sulfide current at a temperature of100° to 450° C.
 3. The process of claim 1, wherein at least one of saidfirst and second catalysts is not silica-selectivated.
 4. The process ofclaim 1, wherein at least one of said first and second catalysts issteam-treated prior to said sulfiding treatment.
 5. The process of claim1, wherein at least one of said first and second catalysts comprises ahydrogenation component in the reduced state prior to said sulfiding. 6.The process of claim 1, wherein at least one of said first and secondcatalysts is/are sulfided by exposure to hydrogen sulfide at atemperature of from above room temperature to 540° C. for a period oftime sufficient to provide sulfur in an amount of at least 0.5equivalents based on the catalyst metal content.
 7. The process of claim1, wherein each of said first and second catalysts comprises anintermediate pore size molecular sieve and characterized by a ConstraintIndex within the range of 1 to 12, independently selected from the groupconsisting of ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38,ZSM-48, ZSM-57, ZSM-58, and mixtures thereof, and wherein said first andsecond hydrogenation components are each independently selected fromGroups 6-11 and 14-15 of the Periodic Table.
 8. The process of claim 1,wherein said at least one first catalyst has a higher alpha value thansaid at least one second catalyst.
 9. The process of claim 1, whereinsaid at least one first catalyst is in a first bed and said at least onesecond catalyst is in a second bed and wherein saidethylbenzene-depleted aromatic hydrocarbon mixture is cascaded from thefirst bed to said second bed without intervening separation of lightgases.
 10. The process of claim 1, wherein said at least one firstcatalyst has a crystal size of greater than 1 micron and said at leastone second catalyst has a crystal size of from 0.02 to 0.05 microns. 11.The process of claim 1, wherein said first and second hydrogenationcompounds are rhenium.
 12. A reactor system comprising, in sequence andin separate but fluidly connected beds, a first catalyst having anaverage crystal size of greater than 1 micron and a second catalysthaving an average crystal size of 0.02 to 0.05 microns, wherein each ofsaid first and said second catalysts comprise a molecular sieve and asulfided hydrogenation component, wherein each of said molecular sievesis not silica selectivated and is independently selected from the groupconsisting of ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38,ZSM-48, ZSM-57, ZSM-58, and mixtures thereof, wherein said firstcatalyst has a higher alpha value than said second catalyst, and whereinsaid first catalyst is in contact with a first C8 aromatic hydrocarbonmixture and said second catalyst is in contact with a second C8 aromatichydrocarbon mixture.
 13. The reactor system of claim 12, wherein saidhydrogenation components on said first and second catalyst areindependently selected from Groups 6-11 and 14-15 of the Periodic Table.14. The reactor system of claim 12, wherein said hydrogenation componentis rhenium and said molecular sieve is ZSM-5, in both said first andsecond catalysts.
 15. The reactor system of claim 12, wherein saidhydrogenation component is rhenium.